Heat-treating aluminum foil



Patented May '2, 11950 UNITED STAT HEAT-TREATING ALUMINUM FOIL AlexanderG. Jarvieand Cyril H. Hannon, Pittsfield, Mass, assignors to GeneralElectric Company, a corporation of New York N Drawing. Application March3, 1948, Serial No. 12,897

1 Claim.

This invention relates to the heat treatment of aluminum foil, either inroll or stacked form,

in an atmosphere and, more particularly, in an atmospher designed toprevent sticking of adjacent layers of aluminum foil during and afterheat treatment.

One of the steps in the manufacture of aluminum foil is an annealingoperation which is used to impart ductility to the foil and to removelubricants from the surface of the foil. The lubricant, which maycomprise a hydrocarbon oil, a metallic soap solution or the like, isapplied to the foil to control friction and heat generated during therolling process. The ordinary processes of annealing have not beensuccessful in removing all of the lubricant Without resulting in theoxidation of the lubricant and the aluminum surfaces. The net result isthat the layers of the foil stick together, thus resulting in severelosses during subsequent unwinding and rewinding. This result isparticularly objectionable in the subsequent use of the annealed foil inthe manufacture of capacitor rolls Where the winding of such rolls iscarried on in high speed Windin'g machines. In such case any tendency ofthe foil to stick seriously interferes with the winding operation andcauses excessive breaking and uneven winding of the foil.

We have discovered that the sticking of aluminum foil during theannealing process may be prevented by heating it in an atmosphere ofcontrolled composition, characterized by specified limits on thepercentage of the gas constituents. We have discovered that theatmosphere used may be reducing, neutral, or oxidizing in chemicalnature, to the limited extent of certan prescribed percentages of thegas constituents.

According to our invention th controlled atmosphere comprises not lessthan 70% by volume of nitrogen, not more than by volume of carbondioxide, not more than 1% by volume of oxygen, less than 2.1 by volumeof water vapor, the remainder comprising reducing gases.

It is, therefore, the primary object of this invention to provide anatmosphere in which the annealing of aluminum foil is carried out in anatmosphere comprising primarily nitrogen in a major proportion andreducing gases, oxidizing gases, or both in definitely limitedquantities.

As an example of our invention, an atmosphere having the followingcomposition has been found to be satisfactory for annealing aluminumfoil in stacked or coil form whereby the foil is prevented from stickingduring and after the annealing operation:

Not less than by volume of nitrogen Less than 1% by volume of oxygenLess than 2.1% by volume of water vapor Not over 10% by volume of carbondioxide The remainder comprising reducing gases, such as hydrogen,carbon monoxide and methane.

While th method of preparing the above atmosphere is not critical to thepresent invention, such an atmosphere can be obtained by incompletelyburning a hydrocarbon mixture such as city gas in a reaction chamber. Byvarying the proportions of air and gas burned in a manner wellunderstood by those skilled in the art, the composition of the resultinggases coming from the reaction chamber may be controlled so as toproduce any desired composition within the range set forth above. Wherethe ratio of air to gas is such that nearly perfect combustion isobtained the resulting gas becomes strongly oxidizing in nature. Thus,if nearly all the hydrogen, carbon monoxide and methane are consumed,the resulting atmosphere will be high in nitrogen and will also containmore than 10% carbon dioxode. On the other hand, if such gaseous mixtureis partially burned by using a smaller amount of air, the nitrogen andcarbon dioxide content will be lower. For the purposes of our invention,it is essential that the nitrogen content should not drop below 70% byvolume of the total atmosphere. It is also necessary that the oxygencontent of the atmosphere be maintained within predetermined limits orsticking of the foil will result. As previously stated, the oxygencontent should be less than 1%, preferably in amounts of about 0.6%.This sticking of the foil also accounts for the 10% limitation on thecontent of the carbon dioxide as well as the limitation on the amount ofwater vapor that may be present.

In racticing the method of our invention, with the view in mind ofpreventing the adjacent layers of the aluminum foil from sticking duringand after the annealing step, the aluminum foil is placed in an enclosedspace and enveloped in a controlled atmosphere as outlined above. Thisatmosphere is maintained in the enclosed spaced during the entire periodof the annealing operation. After the annealing operation is completedthe aluminum foil is allowed to cool and may then be removed from theheating chamber. No further treatment of the aluminum foil is necessaryto facilitate its being unwound and rewound into capacitor rolls withoutthe interlayer sticking that has been so prevalent in the processesemployed prior to our present invention.

It is to be understood that the above description is merely illustrativeand that the invention is not to be limited in any respect except asdefined in the following claim.

What we claim as new and desire to secure by Letters Patent of theUnited States, is:

The method of preventing interlayer sticking during the heat treatmentofaluminum foil in coil form which comprises heating the coil :atannealing temperatures in a closed chamber in an atmosphere consistingof the incomplete combustion products of a mixture of gaseoushydrocarbons, said atmosphere consisting by volume of not less than 70%nitrogen, not more than 10% carbon dioxide, not more than 1% oxygen,less than 2.1% water vapor and the remainder a mixture of the reducinggases, carbon monoxide, hydrogen and methane.

ALEXANDER G. JARVIE.

CYRIL H. HANNON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES "PATENTS Name Date Schon Feb. 13, 1940 OTHER REFERENCESControlledntmospheres for the Heat Treatment ofMetalspages 93, 94, and282. Edited by Jenkins. Published in 1946 by Chapman and Hall, Ltd,London, England.

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